The present invention relates to a method of treating the incineration ash containing the heavy metal and the wastewater sludge, and more particularly, to a method comprising the steps of making the heavy metals existing in the incineration ashes insolulable, stabilizing the organic materials and separating from the ashes, and burying them in the conventional structure used for burying the plants. And such process is based on the reasonable plan considering the mutual relations between the properties.
The method of disposing the solid wastes recently causes an additional problem of city life since the method depends mainly on burying them under the ground. However, it is scarcely possible to secure a vast area of ground enough to bury the wastes even in the countries having the small territory in area such as Korea. Therefore, as an alternative plan, a method of treating the wastes by combusting them has been proposed for extending the effective life of the ground to bury in.
In case that the wastes are incinerated and buried in the ground, an advantage that the effective life of landfill may be extended by reducing the volume of trash to be buried may be achieved.
Generally, there have been proposed two types of methods of treating the incineration ash, one of which is the conventional solidification method carried out by mixing the solidifier and the incineration ash together with the additional water in a mixer, and then solidifying the mixture. And a chemical method of solid stabilization by adding a certain chemical agent to the incineration ash to stabilize the heavy metals existing in the ash.
However, the conventional solidification method has several defects as follows:
1) The effect of reducing the volume of incineration ash is not significant, 2) since the heavy metal in the cement may be eluded out depending pH value, the possibility of elution of the heavy metal is great, and 3) the cost for treatment is also high. The chemical method also has the defects that: 1) the expensive chemicals (chelating agents) should be used; 2) another possible problem causing another contamination may be caused due to using those chemicals (chelating agents); and 3) the possibility of elution of the heavy metal depending on pH is still high.
It is shown in the FIGS. 1 and 2 attached herein that the defects of the prior art, depict the amount of heavy metal contained in the incineration ash and the amount of elution, respectively. And FIG. 2 shows the result of elution test carried out by TCLP method.
FIG. 1 shows the amount of total contents of heavy metal, which the heavy metal contained in the incineration ash comprises 15.936 mg/kg of Pb, 620 mg/kg of Cd, 1.662 mg/kg of Cu, 20.311 mg/kg of Zn and so forth, which is relatively great amount.
Furthermore, as a result of analyzing the elements of incineration ash, it was detected that the incineration ash contained little nitrogen (N) and up to 3.997% of sulfur.
FIG. 2 shows the amount of elution of heavy metal, which the heavy metal eluded from the incineration ash comprises 12.51 mg/l of Pb, 17.03 mg/l of Cd, 5.90 mg/l of Cu, 302.19 mg/l of Zn etc., which significantly exceeds the current standard allowed by the government.
As mentioned above, a little amounts of heavy metals incinerated is not essentially removed from the ash during the incineration process but it is just being concentrated and still existing in the incinerated ash (as being the fly ash as well as the ash lain on the bottom). And if entrapped by the dust collector, its concentration becomes much higher than that has not gone through the incineration. The heavy metals contained in the fly ash and the ash lain on the bottom may be eluded by the rain water when filled in the landfill, and become the leachate, which may cause the secondary contamination of soil and underground water Therefore, a suitable control and/or treatment are required in order to avoid such contamination.
It is therefore the object of the present invention to provide an essential method for treatment of the incineration ash, which may solve above problems
The present invention provides a method of treating the incineration ash and the wastewater sludge (hereinafter, called as xe2x80x9cincineration ashxe2x80x9d), both of which contain a certain amounts of heavy metals, which comprises the steps of converting the heavy metals contained in the incineration ash into an insoluble state, using a safe manner, in order to prevent the exuding of the heavy metals from the incineration ash outwardly when eluded.
The present invention also provides a method for treatment of the incineration ash by burying together with together with organic wastes, which comprises treating the incineration ash with sulfate-reducing bacteria, which may be provided by the organic wastes containing the sulfate-reducing bacteria.
To accomplish above object, the present invention provides a method for treatment of the incineration ash and the wastewater sludge, wherein a mutual relations in using nutrients exists between the incineration ash containing the heavy metal and the organic wastes containing the wastewater sludge and sulfate-reducing bacteria, which comprises: burying the incineration ash together with the organic wastes under the ground, in which the bacteria reduce the sulfates existing in the incineration ash to form the sulfides, and binding the formed sulfides with the heavy metal to form the insoluble metal sulfides, in order to prevent the heavy metal from exuding out of the incineration ash as eluate.
The present invention also provides a method for treatment of the incineration ash and the wastewater sludge, comprising: burying the incineration ash together with the organic wastes containing the sulfate-reducing bacteria under the ground, and covering them with a depth of soil, so that the said sulfate-reducing bacteria promote the heavy metal to precipitate in the form of insoluble metal sulfides, in order to prevent the heavy metal from exuding out of the incineration ash as eluate.
The present invention further provides a method for treatment of the incineration ash and the wastewater sludge, which comprises: collecting and mixing the aqueous eluate exuding out of the incineration ash and the aqueous eluate exuding out of the organic waste containing the said sulfate-reducing bacteria in a disposal plant, and reacting both the aqueous eluates so that the said sulfate-reducing bacteria existing in the aqueous eluate exuding out of the organic waste deposits the heavy metals from the aqueous eluate exuding out of the incineration ash as the insoluble metal sulfides, resulting in the disposal of the aqueous eluate exuding out of the incineration ash and the aqueous eluate exuding out of the organic waste.
Furthermore, according to an aspect of the invention, the said organic wastes may comprise the sewage sludge containing the said sulfate-reducing bacteria.
The present invention will be described hereinafter by referring to attached drawings.
The present invention provides a method for treatment of the incineration ash and the wastewater sludge, which is carried out by reducing the large amount of sulfates in the inclination ash into the sulfides, binding the sulfides with the heavy metals from the inclination ash, and precipitating and removing the insoluble metal sulfides as formed.
FIG. 3 shows the result of analysis for the elements of the incineration ash by use of element analyzer. As seen from the FIG. 3, a large amount of sulfur element exists in the incineration ash.
The element sulfur binds with oxygen in the incineration ash, and exists in the form of sulfate (SO4xe2x88x922) in the solution containing the incineration ash.
The mechanism of binding the heavy metal with sulfur in the incineration ash by the sulfate-reducing bacteria will be explained hereinafter referring to FIG. 4.
FIG. 4 shows the pathway through which the sulfate (SO4xe2x88x922) is to be reduced into sulfides (Sxe2x88x922) by the sulfate-reducing bacteria.
The sulfate (SO4xe2x88x922) in the incineration ash is reduced to the sulfides (Sxe2x88x922) by the sulfate-reducing bacteria, and the sulfides bind to the elemental heavy metal to form the precipitate including the insoluble metal sulfides.
In general, the sulfate-reducing bacterium is a kind of anaerobic bacterium, and produces the energy by utilizing lactic acids, resulting in the formation of acetic acid and so forth as the metabolic products. According to this manner, the sulfate-reducing bacteria reduce the sulfates through the anabolic pathway for decomposition of organic compounds.
Furthermore, the sulfate-reducing bacteria can also decompose fatty acids such as palmitic acid as well as the aromatic compounds such as benzoic acid or phenol.
A variety of the sulfate-reducing bacteria existing in the ecosystem on earth require a large amount of energy for decomposing the stable salt such as sulfate (SO4xe2x88x922), and take its role as the final decomposer of the organic compounds likely the methane bacterium in the ecosystem.
In other words, the sulfate-reducing bacteria reduce the sulfate (SO4xe2x88x922) as being the inorganic compound, which acts as the electron receptor under the anaerobic condition in the biological environment, to the sulfides (Sxe2x88x922, HS or H2S), wherein the organic compounds as being used as the electron donor are oxidized. And, the formed sulfides (Sxe2x88x922, HS or H2S) and the heavy metal in the incineration ash bind together due to having the attraction between them, and precipitated.
Since the metal sulfides formed during the process of reducing sulfates are very stable due to having the solubility significantly lower than metal oxides, metal hydroxides or metal carboates, there is no risk of re-elution. Most heavy metals can form the metal sulfides through the reaction with sulfates under the general anaerobic condition. In particular, even Mn, Zn and Al and so forth, which hardly precipitates as the forms of hydroxides or carboates, may react with the sulfides, to form a stable compound and to be stabilized.
The heavy metals may exist in the form of salts or simple substances in the landfills of trash or wastes. The heavy metals existing as the dissolved form in the landfills of trash 4or wastes may move as the ionic state, and such pathway for the heavy metal to be insolubilized may be called to comprise the steps of adsorption and precipitation. The adsorbed heavy metals can move again when the adsorption reaches the saturation point or the adsorbents are decomposed. Therefore, it may be mentioned that the final insolubilization of heavy metals in the landfill is mainly carried out by the precipitation.
The metal sulfides formed by binding sulfides and heavy metals may be the stable compounds in the solid form, unless being affected by the external condition such as aerobic environment or low pH value. The reason is that since the solubility of metal sulfides is extremely low, the heavy metal scarcely eluded from the metal sulfides. In the meantime, the landfills filled with the trash and/or wastes usually provide the anaerobic or almost anaerobic condition and pH ranging from 6 to 10. Therefore, once the metal sulfides formed, they are not re-eluded.
The solubility products of metal sulfides at 25xc2x0 C. are provided in Table 1 as shown below:
As seen from Table 1, it is found that most of all metal sulfides remain as the almost stable compounds in solid form, and the elution occurs only in extremely few cases.
FIG. 5 shows the insolubility of heavy metals caused by the sulfate-reducing bacteria. The initial concentrations of the heavy metal were set as follows: 20 mg/l of Hg, 10 mg/l of Cd, 10 mg/l of Pb, 10 mg/l of Ni, 5 mg/l of Cu and 10 mg/l of Mn
According to the result of experiment, within a quarter of a day, more than 50% amounts of most heavy metal was removed by the sulfate-reducing bacteria, and in particular, in case of heavy metals such as Cu, Hg, Pb or Cd, 99% or more amounts of heavy metals were removed.
The result of experiment shows that the heavy metals contained in the incineration ash can be insolubilized by precipitating them as metal sulfides. As a result, it is expected that the possible contamination of soil and underground water due to the elution of heavy metal in the treatment of incineration ash can significantly be decreased.
The present invention uses the sulfate-reducing bacteria in order to solidify the harmful heavy metal contained in the incineration ash into the precipitates in the stable form of metal sulfides. In this regard, since like the most of organism, the sulfate-reducing bacteria also use the energy, which has been obtained during the anabolism such as the decomposition of organic compounds, a certain organic material should also be provided in order to utilize the metabolism of the sulfate-reducing bacteria. Since the sulfate-reducing bacteria use the various organic compounds as the carbon source, they are in the advantageous position relative to the other microorganisms in an aspect of competition for securing the organic compounds.
Since the sulfate-reducing bacteria is an anaerobic or at least almost anaerobic microorganism, an anaerobic or at least almost anaerobic condition should be provided for the sulfate-reducing bacteria to grow in the said environment. As a matter of fact, the sulfate-reducing bacteria may be distributed to a high density in an organic waste in which an anaerobic condition of ecological environment has been set up.
The representative examples of the organic waste in which the sulfate-reducing bacteria exist include the sewage sludge, the drainage sludge, the dirty riverbed sludge and so forth (hereinafter, called as xe2x80x9csewage sludgexe2x80x9d).
The reason for the sulfate-reducing bacteria to exist in the sewage sludge to a high density is the anaerobic condition provided by the sewage sludge as well as the sufficient amount of organic compounds contained in the sewage sludge, which the sulfate-reducing bacteria decompose in order to obtain the energy.
Table 2 was provided to explain the latest fact, in which the result of analyzing elements sampled from the sewage sludge obtained from the various sewage disposal plants.
As seen from Table 2, when seeing the resulting data of analyzing elements sampled from the sewage sludge, it was found that relatively large amount of carbon element (17 to 45%) and nitrogen (2.5 to 5.0%) were contained in the sludge. Furthermore, although not provided to Table 2, the amount of contents as well as that of eluent of heavy metal is very low in the sewage sludge. Therefore, it may be found that the sewage sludge has the condition for growing the sulfate-reducing bacteria.
Table 3 shows the compared density of the sulfate-reducing bacteria existing in sewage sludge from the various provinces in Korea.
As seen from Table 3, when seeing the resulting data of analyzing elements sampled from the sewage sludge, it was also found that relatively large amount of sulfate-reducing bacteria were contained in the sludge. However, no sulfate-reducing bacterium exists in the incineration ash.
An alternative example of the organic waste containing the sulfate-reducing bacteria includes the soil from the paddy field. The sulfate-reducing bacteria exist as mainly being a form of condensed accumulation sizing from several tens to hundreds of xcexcm in the soil of paddy field, and exist as a form of condensed accumulation in stream bed.
Furthermore, An alternative example of the organic waste containing the sulfate-reducing bacteria includes the landfills of trash and/or wastes.
Table 4 provides the result of determining the density of sulfate-reducing bacteria existing in the trash and/or wastes filled in Nanji-do landfill.
As shown in Table 4, not a little amount of sulfate-reducing bacteria was distributed in the trash/wastes buried in Nanji-do landfill, up to 105 MPN/ml. Furthermore, although not shown in Table 4, not a little amount of sulfate-reducing bacteria was also distributed in the trash/wastes from the other landfills.
Aforementioned, the reason that not a little amount of sulfate-reducing bacteria exists in the sampled trash/wastes from the landfills is that the landfills of the trash/wastes adjust the anaerobic condition, and the various kinds of organic material exist in a mass
According to another aspect of the present invention, a method is provided for removing the heavy metal in the incineration ash by using the sulfate-reducing bacteria existing in the natural system with the anaerobic condition of the ecological environment, which includes the organic wastes. In other words, the present invention uses sulfur component contained in the incineration ash, and the organic material and the sulfate-reducing bacteria contained in the organic wastes, in order to treat the harmful heavy metal in the incineration ash in a stable manner.
The propagation of sulfate-reducing bacteria in the incineration ash and the reduction of sulfates may be explained by the following experiments.
Considering that the common landfills exhibit at least almost anaerobic or fully anaerobic condition with a low level of Oxidation-Reduction Potential (ORP), each 50 grams of bottom ash and fly ash and 500 ml of distilled water were introduced into a 1000 ml volume of container, and 4 g of sewage sludge were added. After reducing by use of CO2 gas and sealing, the bacteria in the container were cultured at 30xc2x0 C. After 2 to 3 days, ORP value was determined as being xe2x88x92300 mV or below, and the color was changed to red, which represents the propagation of sulfate-reducing bacteria.
Furthermore, it was reported that the addition of aqueous soil extract containing the sulfate-reducing bacteria to the incineration ash in a glass column tube promote the reduction of sulfates.
Therefore, since the sulfate-reducing bacteria can actively act in the common landfills exhibiting at least almost anaerobic or fully anaerobic condition with a low level of Oxidation-Reduction Potential (ORP), a co-treatment of the incineration ash and the sewage sludge fulfills the said condition.
As mentioned above, in a treatment of the incineration ash, the use of organic wastes as the sulfate-reducing bacteria may result in a synergic effect enabling the co-treatment of heavy metal as well as organic material in the organic wastes. Furthermore, it is possible to increase the ratio of removing the heavy metals in the incineration ash by the anaerobic condition formed by the sulfate-reducing bacteria as well as the other kinds of bacteria existing in the organic wastes and decomposing the organic compounds. Furthermore, the organic wastes themselves can also be stabilized due to the active anabolism of sulfate-reducing bacteria.
In other words, in case that the incineration ash (sulfates+heavy metal) and the organic wastes (organic material+sulfate-reducing bacteria) containing the sulfate-reducing bacteria are co-treated, an effect of insolubilizing the heavy metal and removing the organic material may result, and it may be applied to the treatment of a hardly decomposing organic material or the wastewater containing the heavy metal and the wastewater sludge.